Knocking in an internal combustion engine is sensed by sensors which are responsive to oscillations at particular locations in the engine. The sensors utilized include pressure sensors, force sensors or acceleration sensors. The signal furnished by the sensor generally includes a background noise signal which is generated during normal operation of the engine even in the absence of knocking. This background noise varies generally in direct dependence upon engine speed and load. Since the desired signal indicative of engine knocking has a particular frequency, this must be filtered out of the signal including the background noise. Such apparatus is described in German Published Patent Application DE-AS No. 24 45 067. In the apparatus disclosed in this German publication, a correction signal is subtracted from the total sensor output signal. The subtraction signal corresponds to the average intensity of the background noise before the start of engine knocking. The correction signal is generated by creating an average value throughout a relatively long-time interval, so that errors due to momentary variations are eliminated. U.S. Pat. No. 3,822,583 describes a method and apparatus for measuring knocking in an internal combustion engine and for determining the octane value. In this apparatus also, the sensor furnishes a signal which includes both background noise and the desired knock signals. The latter are filtered in the knock frequency range and are then amplified. The sensor output signals, including the knock signals, are compared to a level indicative of the average background noise, the positive differences integrated and then used as a control signal for determining the ignition timing. All of the above-described systems and methods have the common disadvantage, that malfunctioning and, in particular, a total lack of operation of the sensor cannot be recognized. A complete lack of output signals from the sensor resembles similar signals which are generated when the engine is not knocking. A corresponding regulation of ignition timing would then cause the engine to be driven even further into the knock region.